Write file content obeying whitespace using expl3

Question

I want to create a control sequence using expl3 to write file content while obeying whitespace. So far I have the following MWE which does write to file, but ignores whitespace.

\documentclass[]{article}

\usepackage{expl3}

\ExplSyntaxOn

\iow_new:N \g_mwe_stream

\cs_new:Nn \write_whitespace:n {
  \iow_open:Nn \g_mwe_stream { mwe.txt }
  \iow_now:Nn \g_mwe_stream { #1 }  
  \iow_close:N \g_mwe_stream
}

\write_whitespace:n {
abc def
123 456
}

\ExplSyntaxOff

\begin{document}
\end{document}

The MWE creates the file mwe.txt which contains

abcdef123456

instead of the desired

abc def
123 456

I know I can use ~ and ^^J to manually create spaces and line breaks, but I want to avoid this. In LaTeX2 there is the \obeyspaces and \obeylines macros, but I have not found equivalents for LaTeX3.

Answer 1

You should do the writing outside the scope of \ExplSyntaxOn.

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn

\iow_new:N \g_slo_output_stream
\seq_new:N \l__slo_output_seq

\NewDocumentCommand{\writefile}{O{\\}mm}
 {% #1 = line separator, #2 = file name, #3 = text
  \iow_open:Nn \g_slo_output_stream { #2 }
  \seq_set_split:Nnn \l__slo_output_seq { #1 } { #3 }
  \seq_map_inline:Nn \l__slo_output_seq
   {
    \iow_now:Nn \g_slo_output_stream { ##1 }
   }
  \iow_close:N \g_slo_output_stream
}

\ExplSyntaxOff

\writefile{\jobname.txt}{
  abc def \\
  123 456
}

{\obeylines
\writefile[^^M]{\jobname.tx2}{% <-- don't forget
  abc def
  123 456
}}

\stop

I recommend not using the second way.

The first file will not have an empty final line. Apart from this, they'll contain

abc def
123 456